Not to mention the fact that by the time the Ki84, the Ki-61-II, and the Ki-100 entered testing and service, the fuel quality used by the Japanese was so poor, the engines were not producing no where close to the rated horsepower numbers that you are using for comparsion.
For instance the Ki-84 rated at 1,800hp to 2,000hp, depending on engine model, tested at 392 mph at 20,000ft using Japanese fuel during the war. Just after the war when tested in the US, at combat configuration with 100 octane fuel it made 427mph at the same altitude (FYI faster than the Mustang at the same altitude).
So basically your not comparing apples to apples.